1. Field of the Invention
The present invention relates to a height measurement device and to a height measurement method, and in particular to an improvement for a portion for adjusting an optical focal point.
2. Related Arts
A plurality of bump terminals, employed as input/output terminals, are arranged in a matrix on a surface of a semiconductor chip and the semiconductor chip is bonded face down to a substrate. Such a semiconductor chip is called a flip-chip, and is employed for a multi-chip module.
Generally, the bump terminals, which are spherical and are formed on electrodes on the surface of a semiconductor substrate, are, for example, soldering bumps formed by a soldering dip method, or metal bump terminals formed by a plating method.
When a chip on which a plurality of bump terminals are formed is bonded face down to a substrate, the individual bump terminals must be correctly aligned with corresponding electrodes on the substrate. Therefore, the bumps on the chip must have a uniform size and height. If the height of one of the bumps were to be less than that of the others, a defective connection would occur when the bumps were brought into contact with the electrodes on the substrate. Likewise, if the height of one bump were to be greater than that of the others, that bump would be larger than the others, and it could cause a short-circuit by contacting adjacent bumps during the bonding procedure.
Generally, therefore, the heights of the bumps on the surface are examined before a flip-chip is bonded to a substrate. The common height inspection method is a triangulation method whereby to detect the heights of bumps the surface of a semiconductor substrate is irradiated with light, and the reflected light forms an image on a split photodiode or a PSD (Position Sensitive Detector) to detect the heights of the bumps. Thus, a laser beam scans the surface and the heights of the bumps are determined in accordance with the locations from which light is reflected.
FIG. 23 is a diagram showing the principle of the triangulation method. An example for examining the height of a bump terminal 2, which is an input/output terminal formed on the surface of a semiconductor chip 1, an object to be measured. A laser beam 7, an irradiated light, passes through a focusing lens 3 on the irradiation side and forms an image either on the top of the bump 2 or on the surface of the chip 1, which is the object to be measured. Light reflected from the object to be measured passes through a focusing lens 4 on the light reception side and forms an image on a PSD (Position Sensitive Detector) 5, which is a light point position detection device. The height of the bump terminal 2 is detected by employing the relationship between an image forming point 2a on the PSD 5 when the top of the bump terminal 2 is irradiated and an image forming point 1a when the surface of the chip 1 is irradiated.
To increase the accuracy of the height measurement, however, the focus of the optical system on the irradiation side must be so adjusted that the laser beam used for irradiation forms an image on the surface of an object placed at the focal point. This is especially true since the top of the bump terminal is spherical and if the focal point is not on the top, a light point image formed on the top is blurred, so that the quantity and the density of reflected light are reduced. A reduction in the light quantity degrades the detection accuracy of the PSD 5.
Similarly, the focusing of the optical system on the reflection side must be so adjusted that the surface of the PSD 5 becomes the focal point for the reflected light. This is important because the light quantity and density at the point where light forms an image on the surface of the PSD 5 are increased and the sensitivity of the PSD 5 is thereby enhanced.
Therefore, conventionally, a stage on which an object to be measured is mounted, a focusing lens and a PSD are arranged at positions acquired from the specifications for a lens, etc., to constitute an optical system. With this method, however, the focal point is slightly changed because it is shifted due to lens crossovers and because of the arrangement provided less accuracy, and it is difficult to accurately measure the height of a bump terminal.